Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of soft or easily deformed materials including but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, and laminates. Coarse surfaces and stretchable polymeric films are economically printed using flexography.
Flexographic printing members are sometimes known as “relief” printing members (for example, relief-containing printing plates, printing sleeves, or printing cylinders) and are provided with raised relief images onto which ink is applied for application to a printable material. While the raised relief images are inked, the relief “floor” should remain free of ink. The flexographic printing precursors are generally supplied with one or more imagable layers that can be disposed over a backing layer or substrate. Flexographic printing also can be carried out using a flexographic printing cylinder or seamless sleeve having the desired relief image. These flexographic printing members can be provided from flexographic printing precursors that can be “imaged in-the-round” (ITR) using either a photomask or laser-ablatable mask (LAM) over a photosensitive composition (layer), or they can be imaged by direct laser engraving (DLE) of a laser-engravable composition (layer) that is not necessarily photosensitive.
Flexographic printing precursors having laser-ablatable layers are described for example in U.S. Pat. No. 5,719,009 (Fan), which precursors include a laser-ablatable mask layer over one or more photosensitive layers. This publication teaches the use of a developer to remove unreacted material from the photosensitive layer, the barrier layer, and non-ablated portions of the mask layer.
There has been a desire in the industry for a way to prepare flexographic printing members without the use of photosensitive layers that are cured using UV or actinic radiation and that require liquid processing to remove non-imaged composition and mask layers. Direct laser engraving of precursors to produce relief printing plates and stamps is known, but the need for relief image depths greater than 500 μm creates a considerable challenge when imaging speed is also an important commercial requirement. In contrast to laser ablation of mask layers that require low to moderate energy lasers and fluence, direct engraving of a relief-forming layer requires much higher energy and fluence. A laser-engravable layer must also exhibit appropriate physical and chemical properties to achieve “clean” and rapid laser engraving (high sensitivity) so that the resulting printed images have excellent resolution and durability.
A number of elastomeric systems have been described for construction of laser-engravable flexographic printing precursors. For example, U.S. Pat. No. 6,223,655 (Shanbaum et al.) describes the use of a mixture of epoxidized natural rubber and natural rubber in a laser-engravable composition. Engraving of a rubber is also described by S. E. Nielsen in Polymer Testing 3 (1983) pp. 303-310.
U.S. Pat. No. 4,934,267 (Hashimito) describes the use of a natural or synthetic rubber, or mixtures of both, such as acrylonitrile-butadiene, styrene-butadiene and chloroprene rubbers, on a textile support. “Laser Engraving of Rubbers—The Influence of Fillers” by W. Kern et al., October 1997, pp. 710-715 (Rohstoffe Und Anwendendunghen) describes the use of natural rubber, nitrile rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and styrene-butadiene copolymer (SBR) for laser engraving.
EP 1,228,864A1 (Houstra) describes liquid photopolymer mixtures that are designed for UV imaging and curing, and the resulting printing plate precursors are laser-engraved using carbon dioxide lasers operating at about 10 μm wavelength. Such printing plate precursors are unsuitable for imaging using more desirable near-IR absorbing laser diode systems. U.S. Pat. No. 5,798,202 (Cushner et al.) describes the use of reinforced block copolymers incorporating carbon black in a layer that is UV cured and remains thermoplastic. Such block copolymers are used in many commercial UV-sensitive flexographic printing plate precursors. As pointed out in U.S. Pat. No. 6,935,236 (Hiller et al.), such curing would be defective due to the high absorption of UV as it traverses through the thick imagable layer. Although many polymers are suggested for this use in the literature, only extremely flexible elastomers have been used commercially because flexographic layers that are many millimeters thick must be designed to be bent around a printing cylinder and secured with temporary bonding tape and both must be removable after printing.
U.S. Pat. No. 6,776,095 (Telser et al.) describes elastomers including an EPDM rubber and U.S. Pat. No. 6,913,869 (Leinenbach et al.) describes the use of an EPDM rubber for the production of flexographic printing plates having a flexible metal support. U.S. Pat. No. 7,223,524 (Hiller et al.) describes the use of a natural rubber with highly conductive carbon blacks. U.S. Pat. No. 7,290,487 (Hiller et al.) lists suitable hydrophobic elastomers with inert plasticizers. U.S. Patent Application Publication 2002/0018958 (Nishioki et al.) describes a peelable layer and the use of rubbers such as EPDM and NBR together with inert plasticizers such as mineral oils. The use of inert plasticizers or mineral oils can present a problem as they leach out either during precursor grinding (during manufacture) or storage, or under pressure and contact with ink during printing.
An increased need for higher quality flexographic printing precursors for laser engraving has highlighted the need to solve performance problems that were of less importance when quality demands were less stringent. However, it has been especially difficult to simultaneously improve the flexographic printing precursor in various properties because a change that can solve one problem can worsen or cause another problem.
For example, the rate of imaging is now an important consideration in laser engraving of flexographic printing precursors. Throughput (rate of imaging multiple precursors) by engraving depends upon printing plate precursor width because each precursor is imaged point by point. Imaging, multi-step processing, and drying of UV-sensitive precursors is time consuming but this process is independent of printing plate size, and for the production of multiple flexographic printing plates, it can be relatively fast because many flexographic printing plates can be passed through the multiple stages at the same time.
In contrast, throughput using laser-engraving is somewhat determined by the equipment that is used, but if this is the means for improving imaging speed, the cost becomes the main concern. Improved imaging speed is thus related to equipment cost. There is a limit to what the market will bear in equipment cost in order to have faster imaging. Therefore, much work has been done to try to improve the sensitivity of the flexographic printing plate precursors by various means. For instance, U.S. Pat. No. 6,159,659 (Gelbart) describes the use of a foam layer for laser engraving so that there is less material to ablate. U.S. Pat. No. 6,806,018 (Kanga) uses expandable microspheres to increase precursor sensitivity.
U.S. Patent Application Publication 2009/0214983 (Figov et al.) describes the use of additives that thermally degrade during imaging to produce gaseous products. U.S. Patent Application Publication 2008/0194762 (Sugasaki) suggests that good imaging sensitivity can be achieved using a polymer with a nitrogen atom-containing hetero ring. U.S. Patent Application Publication 2008/0258344 (Regan et al.) describes laser-ablatable flexographic printing precursors that can be degraded to simple molecules that are easily removed.
Copending and commonly assigned U.S. Ser. No. 12/748,475 (filed Mar. 29, 2010 by Melamed, Gal, and Dahan) describes flexographic printing precursors having laser-engravable layers that include mixtures of high and low molecular weight EPDM rubbers, which mixtures provide improvements in performance and manufacturability.
As flexographic imaging (sensitivity) is improved, the need for print quality and consistency increases. In addition, there is a need to make manufacturing as consistent as possible. Laser-engravable compositions to be compounded tend to have relatively high viscosity, presenting challenges in ensuring excellent mixing of the essential components. This problem is addressed with the invention described in U.S. Ser. No. 12/748,475 noted above by incorporating a low viscosity EPDM rubber into the composition. Compression recovery can then be a challenge because a good compression rate and printability are generally associated with high molecular weight elastomers in relatively high viscosity compositions.
However, there continues to be a need to improve both the sensitivity and manufacturability of laser-engravable flexographic printing precursors using laser-engravable compositions having a suitable viscosity and compression recovery. It would be particularly useful to achieve these advantages using near-IR laser-engraving because of the advantages associated with the use of near-IR lasers compared to engraving using carbon dioxide lasers.
In addition, there is a desire to improve sensitivity, to reduce imaging time, and to increase throughput of an imaging engraving apparatus. Also, there is a desire to achieve flexographic printing plate that will provide relief images with good quality solid areas and dot reproduction even when printing is performed at high speeds.